1. Field of the Invention
The present invention relates to a developing device and an image forming apparatus including the developing device such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus, and more particularly to a developing device using a two-component developer including toner and magnetic particles.
2. Discussion of the Background
In an electrophotographic image forming apparatus such as a copying machine, a printer, a facsimile machine, or other similar image forming apparatus, an electrostatic latent image formed on a latent image carrier is developed with a two-component developer (hereafter referred to as a “developer”) including toner and magnetic particles (hereafter referred to as “magnetic carrier”) and is formed into a toner image.
For example, a developing device using a two-component developer generally includes a casing that accommodates the developer and has an opening facing a latent image carrier, a non-magnetic developer carrier rotatably disposed such that a part of the developer carrier is exposed to the outside through the opening of the casing, and a magnetic field generating device disposed in the developer carrier. The developer accommodated in the casing is conveyed to a developing region facing the latent image carrier by rotating the developer carrier. In the developing region, the magnetic field generating device causes the developer to rise on the developer carrier in a form of a magnet brush. The magnet brush rubs itself against a latent image formed on the latent image carrier, and thereby toner is supplied from the magnet brush to the latent image on the latent image carrier.
In the above-described developing device using the two-component developer, a decrease in the distance between the latent image carrier and the developer carrier in the developing region allows a high image density to be easily obtained and reduces a so-called edge effect. However, when the latent image carrier and the developer carrier are close to each other, the image deterioration such as a so-called “omission of a trailing edge” in which a trailing edge portion of a black solid image or a halftone solid image is omitted, tends to occur.
Hereinafter described is a mechanism presumably causing the omission of the trailing edge of a toner image. The mechanism will be described referring to a developing device employing a so-called negative-to-positive developing method using a two-component developer, for example. In this developing device, magnetic carrier in a developer is positively charged, and toner in the developer is negatively charged. Further, a non-latent image portion on a latent image carrier is negatively charged.
In a developing region in the developing device, a magnet brush carried on a developer carrier approaching the latent image carrier continuously faces the non-latent image portion until the magnet brush arrives at a trailing edge of a latent image portion to be developed. During the movement of the magnet brush, a repulsive force generated between the negative charges of the non-latent image portion and the toner causes the toner to move toward the surface of the developer carrier away from the latent image carrier. Hereafter, this movement of the toner will be referred to as “toner drift.” As a result, when the magnet brush arrives at the trailing edge of the latent image portion, the positively charged magnetic carrier in the magnet brush adjacent to the latent image carrier is exposed to the outside. In this condition, no toner is present on the surface of the magnetic carrier that faces the trailing edge of the latent image portion, and therefore, no toner is transferred from the magnet brush to the latent image carrier at the trailing edge of the latent image portion.
Further, when the magnet brush reaches a position slightly inward of the trailing edge of the latent image portion, and when an adhesion force acting between the toner and the latent image carrier is weak, the toner once adhered to the latent image portion on the latent image carrier may be returned to the magnetic carrier in a tip end portion of the magnet brush due to an electrostatic force. Consequently, the trailing edge portion of the latent image portion adjacent to the non-latent image portion may not be developed, thereby causing the omission of the trailing edge.
To prevent the occurrence of the omission of the trailing edge, a developing device in which a magnetic flux density distribution on a developer carrier in a direction normal to the surface of the developer carrier is limited has been proposed (for example, in the published Japanese patent application 2000-305360, and Japanese patent application No. 2001-007510 <Published application No. US2002/0094216>). The limited magnetic flux density distribution reduces the width of a developing region, or nip width, in the direction of rotation of the developer carrier.
In this developing device in which the width of the developing region in the direction of movement of the surface of the developer carrier is reduced, a time for a magnet brush to rub itself against a latent image carrier decreases, thereby restraining the above-described toner drift in which toner moves from the tip end portion of the magnet brush toward the surface of the developer carrier. Thus, toner is present on the surface of magnetic carrier that faces a trailing edge of a latent image portion, and magnetic carrier in the tip end portion of the magnet brush adjacent to the latent image carrier is not exposed to the outside. Therefore, the toner once adhered to the latent image portion on the latent image carrier does not return to the magnetic carrier in the tip end portion of the magnet brush. As a result, the omission of the trailing edge can be prevented.
On the other hand, there has been a problem of toner scattering occurring in a developer collecting section in a developing device to which the developer carried on the developer carrier is returned. Recently, to obtain a fine and high-resolution image, a small-particulate developer including small-particulate carrier and toner has been widely used. However, as the diameter of the developer is smaller, toner scattering tends to occur.
Specifically, the above-described toner scattering is a phenomenon in which floating toner in a developing device spouts out from a gap between the developer carried on a developer carrier and a tip end portion of a casing of the developing device at the downstream side of a developing region with respect to the direction of rotation of the developer carrier. In a developing device using a two-component developer, the toner scattering tends to occur in the following conditions. That is, a condition in which magnetic carrier carried on the developer carrier is uneven and a large amount of floating toner not sufficiently charged exists, when the magnetic carrier is mixed with non-charged toner at the time of initial setup. In addition, a condition in which a large amount of floating toner not sufficiently charged exists immediately after fresh toner is supplied to a developing device.
To restrain the toner scattering, a toner-scattering restraining sheet-shaped member made of polyethylene terephthalate (PET) is used. The toner-scattering restraining sheet-shaped member is provided on an end portion of a casing of a developing device such that a free end of the toner-scattering restraining sheet-shaped member adjoins a developer carrier to reduce a gap between the casing at a developer collecting section in the developing device and a developer carried on the developer carrier. In the developing device using a two-component developer, the free end of the toner-scattering restraining sheet-shaped member is disposed in a non-contact relation to the developer on the developer carrier so as to prevent the toner-scattering restraining sheet-shaped member from scraping the developer off the developer carrier and prevent the falling of the developer.
FIG. 1 illustrates a developing device in which the width of a developing region, or nip width, in the direction of rotation of a developer carrier is reduced. Specifically, a part of a developing sleeve 4 serving as a developer carrier is exposed to the outside through an opening of a casing 2a facing a photoconductive drum 1 serving as a latent image carrier. A main pole P1 (N pole) for development is formed on a magnet roller 5 in the developing sleeve 4 to cause a developer to rise in the form of magnetic brush at the position facing a developing region (D) formed between the developing sleeve 4 and the photoconductive drum 1.
Further, auxiliary poles P1a (S pole) and P1b (S pole) each having a polarity opposite to that of the main pole P1 adjoin the main pole P1 at the upstream side and downstream side, respectively, in the direction of rotation of the developing sleeve 4. The auxiliary poles P1a and P1b reduce the angular half-width of a magnetic flux density distribution set up by the main pole P1 in the direction normal to the developing sleeve 4. A pole P4 (N pole) is located between a position facing a doctor blade 7a and the developing region (D) such that its magnetic field extends to a developer storing section (S). Further, a pole P2 (N pole) and a pole P3 (S pole) are so positioned as to convey the developer carried on the developing sleeve 4. In FIG. 1, dotted curves around the developing sleeve 4 represent magnetic flux density distributions formed by the poles in the direction normal to the surface of the developing sleeve 4, as measured at the center of the developing sleeve 4 in the axial direction.
As described above, by forming the auxiliary pole P1b adjoining the main pole P1 on the magnet roller 5 to reduce the angular half-width of a magnetic flux density distribution set up by the main pole P1 in the direction normal to the developing sleeve 4, a side of the magnetic flux density distribution formed by the auxiliary pole P1b in the direction normal to the surface of the developing sleeve 4 and close to the main pole P1 is located outside of the opening of the casing 2a. Further, as the size of the developing sleeve 4 reduces, the distance on the developing sleeve 4 between the main pole P1 and the auxiliary pole P1b decreases. As a result, the peak of the magnetic flux density set by the auxiliary pole P1b in its normal direction is located outside of the opening of the casing 2a as well.
In the above-described developing device in which the peak of the magnetic flux density set by the auxiliary pole P1b in its normal direction is located outside of the opening of the casing 2a and the width of the pole P1b is small, a sufficient magnetic force cannot be obtained, and the centrifugal force exerted on the magnetic carrier carried on the developing sleeve 4 by the rotation of the developing sleeve 4 exceeds the magnetic force. As a result, the magnetic carrier may be free of the developing sleeve 4 and scatter. When the magnetic carrier scatters from the part of the developing sleeve 4 exposed to the outside, the magnetic carrier may contaminate the inside of the apparatus. Further, when the magnetic carrier falls on a sheet conveying guide or a transfer sheet, so-called “white spot” occurs. The “white spot” means a condition in which a toner image is partially omitted at around magnetic carrier on a transferred toner image on a transfer sheet. Thus, an image is deteriorated.
Referring to FIG. 6, when a toner-scattering restraining sheet-shaped member 13 is provided on an end portion of the casing 2a of the above-described developing device of FIG. 1 such that its free end adjoins the developing sleeve 4, to reduce a gap between the casing 2a at a developer collecting section in the developing device and a developer carried on the developing sleeve 4, the magnetic carrier scattering from the part of the developing sleeve 4 exposed to the outside falls onto the surface of the toner-scattering restraining sheet-shaped member 13 facing the photoconductive drum 1, and is captured on the toner-scattering restraining sheet-shaped member 13.
Further, in an image forming apparatus, vibrations occur, for example, when a developing device is driven and when a transfer sheet is conveyed. When the magnetic carrier deposited on the toner-scattering restraining sheet-shaped member 13 receives such vibrations, the magnetic carrier may fall from the toner-scattering restraining sheet-shaped member 13. In a developing device which lacks a toner-scattering restraining sheet-shaped member 13, magnetic carrier scattering from a part of a developing sleeve exposed to the outside, specifically the magnetic carrier falling onto a surface of an inner wall of a casing, is returned to the inside of the developing device by the rotation of the developing sleeve. In this condition, if the toner-scattering restraining sheet-shaped member 13 is provided, occurrence of carrier falling increases.